Domestic electric heating appliance



Dec. 24, 1963 J. w. JACOBS DOMESTIC ELECTRIC HEATING APPLIANCE Filed Nov. 25, 1960 INVENTOR. James 14/. Jacob: BY fM ms ATTORNEY 3,l3l5,565 ELECTll-lltll HEATENG Al i lAN-CE James W. Jacobs, Dayton, Ulric, assignor to General Motors Corporation, Detroit, Mich, a corporation of Delaware Filed Nov. 25, 196%, Ser. No. 71536 4 Claims. (Cl. 21l9-2 This invention relates to a domestic appliance and more particularly to an improved temperature sensitive control for a surface cooking unit.

Many of the electric ranges in use today include a temperature responsive control system for at least one of the surface cooking units. The function of such a control is to sense the temperature of any utensil resting on the cooking unit, thereby to control the rate of neat output from the surface cooking unit so that the utensil is maintained at a predetermined temperature. In the prior art two systems have been used. One system utilizes a plurality of bimetallic elements selectively to open and close or pulse a circuit supplying power to the surface cooking unit. Another system utilizes a hydraulic arrangement wherein the temperature of the cooking utensil is reliected through a capillary tube to a control which alters accordingly the amount of power supplied to the cooking unit. Both of these systems tend to get out of calibration easily and are somewhat complex in their construction.

Accordingly, it is an object of this invention to provide a temperature sensitive control for an electric cooking unit which has a simplified construction and an increased sensitivity to temperature fluctuations at the cooking unit.

It is a general object of this invention to provide a temperature sensitive cooking unit control with a variable timing circuit.

It is a more specific object of this invention to provide a temperature sensitive control system for a surface cooking unit with a transistor diode timing circuit including a four layer hyperconductive negative resistance transistor diode for controlling the pulsing rate at which power is supplied to the surface cooking unit.

Further objects and advantages of the present invention will be apparent from the following description, reference being had to the accompanying drawings wherein preferred embodiments of the present invention are clearly shown.

in the drawings:

FIGURE 1 is a fragmentary elevational view of a tubular sheathed heating element suitable for use with this invention; and

FIGURE 2 is a schematic wiring diagram of the control of this invention.

In accordance with this invention and with reference to FIGURE 1, an electric range ill is illustrated as having a conventional tubular sheathed heating element such as 12; supported in an opening M- in the top of the range. The surface cooking unit or heating element l2 is of the temperature responsive type in which a temperature sensing element 16 is positioned in the center of the cooking unit 112 in a manner that will place the element in intimate thermal contact with any utensil placed on the cool:- ing unit 12. A conventional domestic power supply of 230 volts is used to energize the surface cooking unit 12 and it is to the control of this power supply in accordance with a preselected cooking temperature that this invention is directed.

Turning now to FIGURE 2, the temperature sensing control of this invention will now be more clearly described. The temperature sensing element 16 includes a body portion 13 and a cap 2 3 biased upwardly by a spring 22 into intimate thermal contact with the bottom of any utensil (not shown) resting upon the heating eleitcd tates Patent ment 12. To the under side of the sensing element cap 20 there is positioned a thermistor 24 or other suitable device, the resistance of wt ich varies inversely with temperalure. As the temperature of a pan, and thus the food therein, setting on the surface cooking unit 12 increases, the resistance of the thermistor 2 will decrease to pass additional current to a remote master pulsing control unit shown generally at and described more fully next following. The control unit includes a 230-volt power supply circuit and a l15-volt control circuit. In general, the control circuit senses and reflects the pan temperature in a manner to open and close the power supply circuit to the cooking unit, thereby regulating pan temperature.

The master pulsing unit for the temperature sensing control includes a control lznob 28 which may be positioned on the control panel of any conventional electric range. Within the pulsing unit 26 there is disposed a birnctal 3i or other thermally responsive device which is afiixed at one end as at 32. The bimetal 3h carries a first control circuit contact and a first power supply contact The power supply contact 36 cooperates with a second power supply contact 38 which is connected through a conductor of the power supply circuit to the one end of the surface heating element l2the other side of the heating element 12 being connected through a conductor d2 to one side L; of a domestic 230-volt power supply having main line conductors L L and a neutral N.

The control circuit includes a second control contact id which is selectively engaged by the bimetal contact 34 and is thus intermittently connected by Way of a conductor do with a capacitive timing circuit shown generally at 48. The timing circuit 48, which is an unstable cyclic circuitry is conne ted to a rheostat or variable resistance 5% which includes a contact blade or wiper 52 shiftable through a cam 54 on the control knob 28. Any conventional mechanical linkage, such as represented by the dashed line 56, may be used to interconnect the knob and rheostat. Note also that the rotation of the control knob may cause a protuberance 58 to bias the bimetal Z-tl permanently into a completely open position wherein contacts 34 and are disengaged as well as contacts 36 and 38a mechanical linkage on between knob and bimetal serving this purpose. In series with the rheostat fill is a rectifier or diode 62, one side of which is connected to the thermistor 24. At the opposite side of the thermistor a conductor 64 connects the thermistor to the neutral N side of the power supply.

The capacitive timing circuit 48 will now be described more fully. Generally speaking, the object of the timing circuit is to reflect a temperature sensed at the sensing head la and to transpose this sensed temperature into a pulsed signal in any means '7@ which will actuate the bimetal 39. More particularly, the timing circuit includes a heater or resistance 7s and a four-layer hyper-conductive negative resistance transistor diode '72. In parallel with the diode 72. and the heater 74} is a capacitor 74. The parallel timing circuit 48 is thus interposed between the bimetal 3b and the temperature sensing thermistor 24. In accordance with a preset temperature selected by the knob 28, the rheostat wiper blade 52 will be positioned along the rheostat resistance 56 so that the timing circuit ill will be efifcctive to regulate the amount of heat output by the heater '76) to actuate the bimetal.

Since the timing circuit operates on 60 cycles, the capacitor 74 is pulsed with pulsating direct current until a predetermined charge is built up in the capacitor. The four layer diode '72 then discharges the capacitor through the heater. This is a repetitive cycle of short duration which gradually heats the heater 7% to the point where it will operate the bimetal switch to deenergize the heater. The

bimetal switch thus cycles the cooking unit to obtain the desired temperature in the pan.

in other words the bimetal 3th is caused to flex by an actuator or heater it? connected in series with the negative resistance diode '72. ince both are connected in parallel with a capacitor 74, the timing circuit 43 is conditioned to create a signal through the diode 72 whenever the capacitor has taken on a charge suificient to cause an avalanche effect through the diode and the heater The parallel circuit is connected through the rectifier 62 to the alternating current power soruce L L and N an the rectifier is arranged in the reverse relation relative to the diode so that half waves only (DC) are passed to the timing circuit 4-3. The rate of operation of the timing circuit and more particularly the heat output of the heating element 769 is controlled by a resistor or rheostat t? connected in series with the parallel circuit. The foregoing description of the timing circuit is believed sufiicient for the aspects of this invention. However, for additional information relative to the theory of operation and design of the capacitive timing circuit 33, reference may be had to the copending application SiN. 71,581 filed November 25, 1960, and assigned to the same assignee as this invention.

in operation, a utensil is placed on the surface cooking unit 12 in a manner that the cap of the temperature sensing device is is biased into engagement with the bottom thereof. The power is turned on through the rotary knob 2 3-the cam surface 54 biasing a rheostat selector 52 through any conventional linkage 56 to the desired temperature setting. As aforesaid, a positive block or lock out for the bimetal 3t? is aitorded by the linkage 6-9 which is released whenever the knob 28 is turned to any of the cooking temperature settings. At the initiation of a cooking cycle, the bimetal will be cold and in the position shown in FiGURE 2. Thus, power will fiow to the surface cooking unit 12 from L conductor 42, the heating element 12, conductor 4t}, power supply contact 33, power supply contact 36, bimetal 3t? and conductor 7% to the other side of the power supply L As the ingredicnts in the pan heat up, this temperature increase is reflected in the cap 2% and thus the thermistor 24. As the thermistor 24 heats up, its resistance goes down and additional control power is supplied to the timing circuit 48 by Way of conductor 64 and the rectifier as. The amount of DO. power signals passed is governed by the rheostat or variable resistance 5t) preset in accordance with the temperature selection. These signals are charged into the capacitor 7 until the firing voltage of the negative resistance diode 72 is reached. Then the capacitor will discharge through the diode 72 and the heater 70. Thus, a measured amount of heat is transferred from the heater '76 to the bimetal 3%. Depending on the amount of firing pulses through the diode 72, the heater 7d will put out more or less heat and the bimetal will open more or less quickly. As soon as the bimetal 3 flexes to the open position, the power supply circuit to the heating element 1?. is interrupted at the power supply contacts 36 and 38. t the same time, the control circuit to the tim ing circuit 42' is interrupted at the control circuit contacts 34 and Thus, the heating element 12 is deenergized briefly to hold the temperature of the utensil at the selected temperature. As the bimetal so cools, the contacts 44 and 38 are reengaged and the cycle is repeated throughout the cooking operation to maintain the desired temperature setting.

It should now be seen that an improved simplified temperature responsive control circuit for an electric range has been taught by this invention. The control is extremely sensitive and will better retain its calibration throughout the life of the control.

While the embodiments of the present invention as herein disclosed, constitute preferred forms, it is to be understood that other forms might be adopted.

What is claimed is as follows:

1. In combination with an electric cooking unit adapted to support a utensil, a thermally responsive control comprising a thermistor in heat sensing relationship to said utensil, a first and second contact forming a first pulsing switch in circuit with said thermistor, a second and third contact forming a second pulsing switch connected directly to one side of said cooking unit, a single bimetal means for opening and closing said first and second pulsing switches, a heater for actuating said single bimetal means, a negative resistance diode connected in series with said heater, said diode having a predetermined switcling voltage, a capacitor connected in a parallel circuit with the heater and diode, said capacitor being capable of being charged to a voltage great enough to exceed the switching voltage of said diode, a rheostat for controlling the charging rate of said capacitor to regulate said cooking unit, a rectifier connected in series with said thermistor, said rheostat, said parallel circuit and said bimetal means and oriented to prevent flow of current throu a said diode before said diode reaches its switching voltage, and an alternating current power supply having a peak output voltage greater than the switching voltage of said diode and including a first line conductor and a neutral line conductor for supplying energy directly to said single bimetal means and said thermistor for repeatedly energizing said heater and a second line conductor connected directly to the other side of said cooking unit and thereby in series with said second pulsing switch for supplying energy selectively and directly to said cooking unit, said negative resistance diode being in the form of a four layer transistor.

2. In combination with an electric cooking unit adapted to support a utensil, a thermally responsive control comprising a thermistor in heat sensing relationship to said utensil, a first and second contact forming a pulsing switch in circuit with said thermistor, a second and third contact forming another pulsing switch in direct series power supply relationship to said cooking unit, bimetal means for simultaneously opening and closing both of said pulsing switches, a heater adjacent said bimetal means for actuating said bimetal means, a four layer hyper-conductive negative resistance diode connected in Series with said actuator, said diode having a predetermined switching voltage, a capacitor connected in a parallel circuit with the actuator and diode, said capacitor being capable of being charged to a voltage great enough to exceed the switching voltage of said diode, a rheostat for controlling the charging rate of said capacitor to regulate said cooking unit, a rectifier connected in series with said thermistor, said rheostat, said parallel circuit and said bimetal means and oriented to prevent flow of current through said diode before said diode reacaes its switching voltage, and an alternating current power supply having a peak output voltage greater than the switching voltage of said diode and having one conductor connected directly to said bimetal means and another conductor connected directly to said thermistor for supplying energy directly to said himetal means and said thermistor for repeatedly energizing said actuator.

3. In combination with a power source, a range control for a surface unit connected to said power source and adapted to support a utensil comprising, contact means including a first contact connected to said power source and a second contact connected directly to said surface unit, bimetal means for repeatedly automatically operating said contact means to repeatedly directly interrupt power supply to said surface unit, heating means for said bimetal means, and means connected to said heating means for repeatedly periodically energizing said heating means including a capacitive circuit having a capacitor and a negative resistance diode in parallel relationship and a rectifier in series with said capacitive circuit, the

charging rate of said capacitor being automatically varied References Cited in the file of this patent in response to the temperature of said utensil to control UNITED STATES PATENTS the rate of intermittent discharge of said capacitive circuit 1 through said heating means. l2,866,106 Schun 1958 4. The combination of claim 3 wherein said first con- 5 2,947,916 Beck 1960 tact is connected to said power source through a portion 2,970,201 Woodward Ian 1961 of said bimetal means whereby to make said bimetal 2,971,074 stroal at al 1961 means self-heating when said first contact is engaged with 3,013,356 let 1952 3,939,523 Pittman Apr. 17, 1962 said second contact. 

3. IN COMBINATION WITH A POWER SOURCE, A RANGE CONTROL FOR A SURFACE UNIT CONNECTED TO SAID POWER SOURCE AND ADAPTED TO SUPPORT A UTENSIL COMPRISING, CONTACT MEANS INCLUDING A FIRST CONTACT CONNECTED TO SAID POWER SOURCE AND A SECOND CONTACT CONNECTED DIRECTLY TO SAID SURFACE UNIT, BIMETAL MEANS FOR REPEATEDLY AUTOMATICALLY OPERATING SAID CONTACT MEANS TO REPEATEDLY DIRECTLY INTERRUPT POWER SUPPLY TO SAID SURFACE UNIT, HEATING MEANS FOR SAID BIMETAL MEANS, AND MEANS CONNECTED TO SAID HEATING MEANS FOR REPEATEDLY PERIODICALLY ENERGIZING SAID HEATING MEANS INCLUDING A CAPACITIVE CIRCUIT HAVING A CAPACITOR AND A NEGATIVE RESISTANCE DIODE IN PARALLEL RELATIONSHIP AND A RECTIFIER IN SERIES WITH SAID CAPACITIVE CIRCUIT, THE 